1. Field of the Invention
The invention relates to a circuit board on which electronic components can be mounted. In particular, the invention relates to a circuit board having wiring layers that are electrically connected to each other by via conductors.
2. Description of the Related Art
As circuit apparatuses grow in density, circuit boards for mounting electronic components, such as semiconductor devices, have been formed in multiple layers. In general, the wiring layers of a circuit board are electrically insulated from each other by an insulating layer, and are electrically connected to each other by via conductors which pierce through the insulating layer in predetermined positions.
Japanese Patent Laid-Open Publication No. 2004-288795 (hereinafter, referred to as Patent Document 1) discloses a typical example of these conventional circuit boards. The circuit board described in Patent Document 1 has a resin layer that is provided with a glass cloth to ensure strength. Part of the glass cloth protrudes from the inner walls of via holes, thereby establishing an anchoring structure with a plating layer formed in the via holes.
With these conventional circuit boards, the adhesion between the via conductors formed in the via holes and the insulating layer has been inadequate. For example, in the foregoing Patent Document 1, the glass cloth protruding from the sidewalls of the via holes occupies only a limited area in relation to the sidewalls of the via holes, and therefore fails to provide a sufficient anchoring effect.